1. Field of the Invention
This invention relates to a system for detecting a transfer of article, and more particularly a very effective antitheft system for detecting any manual transfer of article(s) disposed on a display stand of a shop.
2. Description of the Prior Art
A conventional antitheft system for detecting any manual transfer of articles disposed on a display stand is proposed in Japanese Examined Patent Publication Ser. No. 52-30836 entitled "Process for monitoring articles and its apparatus" or Japanese Unexamined Patent Publication Ser. No. 60-51758 entitled "A card for the prevention of theft, and its manufacturing process".
According to such a conventional antitheft system, a card with a built-in high permeability magnetic substance is attached previously to a surface of respective articles, thereby signals for generating an alternating field such as microwaves are transmitted to a monitoring area. When transferring an article having the aforesaid card to the monitoring area, higher harmonic wave signals are secondarily transmitted from the built-in magnetic substance of the card by means of the aforesaid signals. Thus, by detecting the higher harmonic wave signals, it is possible to detect a transfer of article to a non-monitoring area, thereby the articles on the display stand can be prevented from being stolen.
However, such conventional detecting system has the following disadvantages and inconveniences in view of performance and shape.
(1) When a certain metal body having a property similar to the magnetic substance of the card is transferred to the monitoring area, the card often malfunctions.
(2) When the card is closely attached to a human body, signal transmission is prevented so that the sensor of the detecting system is inoperative.
(3) If the card is removed from the article before the latter passes out of the monitoring area, it becomes impossible to detect a transfer of such article.
(4) In order to actuate the card stably, it requires a considerably larger size. Accordingly, it is entirely impossible to attach such a large card to respective precious and small articles such as watches, jewels, precious metals or the like.
(5) Even if the detecting system is actuated normally, it becomes difficult to detect any person holding the article with such card from among many people when he or she is lost in the crowd.
In view of the above points, this invention has been achieved.